Acoustic thermography is a non-destructive testing method to prove defects in inspection parts of different sizes, materials and structure.
The inspection part is generally vibrated with high-performance ultrasound (typically >approx. 15 kHz, >approx. 100 W), which, on defects such as cracks or delaminations, is converted into heat by means of different effects. This local thermal response is registered and then evaluated with the aid of a thermal imaging camera. It may however result in local damage in the case of excessive vibrational amplitudes which preferably occur on thinner regions of the inspection part.
The conventional conversion of acoustic thermography takes place with approximately 20 kHz fixed frequency. In this process, the individual vibrational behavior of the individual inspection part is taken into consideration, whereby a relatively poor part-to-part reproducibility results on the one hand and the probability of local damage can neither be predicted nor reduced on the other hand. Crack formation and advances occur as soon as the yield point of the material is locally exceeded.
Attempts can currently be made using the following methods in order to prove that the technology described is non-destructive.
a) Serial measurements on a inspection part, with the risk of damaging a comparable part in a cycle being minimal if the inspection part was not damaged after n testing cycles. Since even components of the same type have slightly different vibrational spectra, it may be that one part is not damaged while another part is however locally damaged under the same test conditions.b) Laser vibrometry examinations for determining the vibration of the inspection part. No statements relating to the loads of inner structures can however be made here.
EP 1 582 867 A2 discloses a method for instance, which operates with attunable exciters, with the aid of which the individual vibrational behavior of each individual inspection part can be taken into consideration. By exciting several resonance frequencies, a better part-to-part reproducibility is achieved at the same time as an increased degree of efficiency, since less electrical power is needed in the case of a resonant excitation in order to achieve an adequately high vibrational amplitude of the inspection part. The disadvantage of possible local damage, for instance at points with a minimal material thickness, still exists however.